Electrodes are necessary elements in devices or applications that employ electric fields or currents to act on liquids. Microanalytical devices that employ electrophoresis, electroosmosis, electrokinetic pumping, and various electrochemical detection schemes pose demanding requirements for electrodes, in particular because these electrodes are required to function in miniaturized reservoirs and/or microchannels.
Electric fields and currents are employed in microseparation systems employing electrophoresis for separation and sample concentration. These include capillary electrophoresis (CE), capillary zone electrophoresis (CZE), capillary electrochromatography (CEC), micellar electrokinetic chromatography (MEKC), and electrokinetically-driven high pressure liquid chromatography (EK-HPLC).
Sample pre-concentration schemes employed prior to separation in microfluidic separation devices have been accomplished by a variety of means employing electric fields. Examples include sample stacking by electrophoresis across a region of non-uniform analyte mobility, isoelectric focusing employing electric fields along pH gradients, and electrophoretic concentration against an analyte impermeable barrier such as a salt bridge or solid-polymer electrolyte. Many miniaturized and conventional-scale analytical systems employ electrochemical detection of analyte species by measurement of the conductivity of the analyte fluid stream.
Moreover, in many of the applications discussed above pressures in excess of 1000 psi can be generated. By way of example, by applying voltage across a porous bed having a charged-solid-liquid interface, electrokinetic pumps are capable of developing pressures in excess of 10,000 psi.
All the applications described above require electrodes to provide the necessary electric fields and currents. The most common electrode in use is simply a metal wire, principally because of both the ease of insertion into the small dimensions of a microfluidic channel and sealing against high pressure. These wires are generally made of nickel, steel, platinum, gold, or other passive or noble metal. The disadvantage attached to the use of these metal wire electrodes in microscale devices, i.e., devices having fluid flow channels ≈1 to 1000 μm, is the formation of gas bubbles, generally arising from the electrolysis of the solvent, e.g., water or formaldehyde. Because of the small dimensions of flow channels in microfluidic devices, these gas bubbles can occlude the current path to the electrodes and introduce electrical noise. In the worst case, the bubbles can block the entire flow channel and thereby open the electrical circuit. In flowing separation or detection systems, electrochemically produced hydrogen or oxygen can react with species of interest and bias or vitiate the analysis. Besides electrolytic gas bubble generation, electroactive species in the fluid being analyzed can be electrochemically reduced or oxidized on the exposed electrodes degrading the analysis and/or upsetting the separation or detection chemistry. In open or low-pressure systems and devices, conducting polymer or salt bridge devices have been used to separate electrodes away from sensitive areas in the device. However, in high pressure systems liquid permeation or loss of mechanical integrity precludes these conventional solutions. What is required is an electrode or electrode device that can be used in high pressure devices to provide the electric fields and currents necessary in the applications described above.